1. Field of the Invention
The present invention relates to a coating composition containing certain chemically modified, amorphous polyolefins, particularly polypropylenes, as coupling agents in combination with certain bisphenol derivatives as a further auxiliary, to a process for coating plastic substrates and to the coated plastic substrates obtained therefrom.
2. Description of the Prior Art
The application of firmly adhering paint coatings to plastics, particularly injection molded articles of polypropylene (PP) requires chemical or physical modification of the substrate surfaces. Flame treatment (cf. for example F. Garbassi et al, J. Mater Sci., 22, pages 207 to 212 (1987)), corona treatment (cf. for example F. Lawson, Rubber Chem. and Technol., 60, 102 (1987)), plasma treatment (cf. for example L. H. Coopes et al, J. Macromol. Chem., 17, pages 217 to 226 (1982) and EP-A-0 282 094), and pickling with chromosulfuric acid (cf. for example M. Salkauskas, J. Appl. Polym. Sci., 26, pages 2097 to 2098 (1981) are mentioned in this regard. It is also evident from the literature that plastics parts based on polypropylene and polyethylenes can be painted with firmly adhering coatings by precoating with coupling agents based on chlorinated polyolefins (U.S. Pat. No. 3,676,391) or polyolefins modified with maleic acid anhydride (DE-OS 1 546 982).
These methods are expensive and, in addition, cannot be applied to injection molded articles of complicated geometry. Apart from the high handling costs, the application of coupling agents involves environmental problems due to their low solids content. In addition, there use is limited to certain paint systems.
In the PCT Application No. WO 88/05346, it is proposed to modify the coatings of coupling agents by additional measures, such as UV irradiation, which involves additional expense.
The incorporation of chlorinated polyolefins, for example conventional chlorinated polyolefins, in the paints, as recommended in U.S. Pat. No. 2,468,480, is attended by the disadvantage that the chlorinated polyolefins show poor compatibility with the paints typically used for plastics. In addition, their solubility in typical paint solvents is very poor which adversely affects processability.
In Applicants' earlier German Patent Application P 39 09 217.8, it is recommended to use chemically modified, amorphous polyolefins - of the type also used in accordance with the present invention - as a coupling agent. However, the sole use of these compounds as coupling agents is attended by the disadvantage that these additives cannot be incorporated in the paints in sufficiently non-sedimenting form such that they soon sediment and thus lose their effectiveness.
Accordingly, an object of the present invention is to provide a new process for painting plastics which, through the use of new coupling agents, would be suitable for the production of high-quality paint coatings without being attended by the disadvantages of the prior art processes cited above. It is a further object to provide coating compositions which contain coupling agents in non-sedimenting form.
These objects may be achieved in accordance with the present invention as described in detail hereinafter by the use coupling agents in combination with certain bisphenol derivatives.